1. Field of the Invention
A squeeze casting method is a casting method wherein, by directly applying high pressure produced by a mechanical means such as hydraulic press to the molten metal in the metal mold cavity, the molten metal is statically pressed to an internal or other space, thereby effecting solidification of the molten metal. The method can produce high quality products by limiting the formation of the casting defect by an application of high pressures during the solidification process. Generally, it is a high pressure casting solidification method which is also denoted by the term "molten metal forging method".
Thus, the squeeze casting method is a composite forming method which combines the press forging method to the solidification shrinkage process where irregular changes in volume occur due to the changes from the liquid phase to the solid phase. In this method, by applying high pressure during the solidification, the molten metal is sufficiently supplied for the shrinkage cavity caused by the solidification shrinkage when the molten metal is solidified during press forming. Further, the pin hole and gas porosities are eliminated by the increase in the dissolution limit of the hydrogen gas, and the molten metal is tightened with the metal mold to limit the formation of the air gap (the empty space formed where the casted metal is separated from the metal mold surface). Therefore, the cooling rate of the cast product is increased, thereby allowing the formation of more fine structures, and thus improving the mechanical properties. Also, this method is a casting method which can produce high density and high quality cast products without any casting defects, such as air gap by injection filling the molten metal into the metal mold cavity at low speeds, thereby eliminating a phenomena such as mixing of the solidified layer or air in the injection sleeve and the metal mold cavity in the molten metal.
2. Description of the Prior Art
The squeeze casting method, according to the method used to apply high pressures to the molten metal during the manufacturing of cast products, can be classified into the three following methods which were known in the prior art. The prior art methods are the direct pressing method (FIG. 5A), the indirect pressing method (FIG. 5B), and the local pressing method (FIG. 5C).
The direct pressing method, as shown in FIG. 5A, is method of directly applying pressure by using an upper pressing plunger to the molten metal poured into the metal mold cavity. This method was most often used when the squeeze casting method was first developed. In this method, the transfer efficiency of the pressure force is very high since the pressure from the pressing plunger is delivered directly to the molten metal inside the metal mold cavity, giving maximum impact to the pressing effect. Thus, solidification occurs rapidly since the tightness between the molten metal and the metal mold is increased. The mechanical characteristics are greatly increased since the production of the metal with a high dense and fine structure are accomplished by eliminating the casting defects caused by solidification shrinkage or air. Also, high dimensional accuracy and good surface profile can be obtained, allowing the manufacturing of high quality cast products. However, the pressing displacement of high pressures is determined by the amount of molten metal poured into the metal mold cavity. In other words, if the poured amount of the molten metal is small, the size of the cast product is less than desired; and if the poured amount of the molten metal is large, the size of the cast product is larger than desired. Thus, it is imperative that the pouring amount of the molten metal be accurate. However, it is very difficult to determine the correct or accurate pouring amount of the molten metal in actual practice. Although it is possible to use an automatic pouring device with a sensor for measuring the height of the molten metal, such a device is extremely expensive.
The indirect pressing method, as shown in FIG. 5(B), is a pressing method where a connecting runner is installed to the metal mold cavity within the metal mold, thereby pouring the molten metal onto a portion of the runner and metal mold cavity, afterwards advancing the upper pressing plunger to inject the molten metal in the runner at low speeds into the metal mold cavity, and thereafter using a pressing plunger to apply high pressure to the molten metal in the runner, thereby effecting solidification of the molten metal in the metal mold cavity. In the indirect pressing method, since extra part of the poured amount collects at the runner, the dimensions of the cast product are not affected by any change in the poured amount and thus, the accuracy in the poured amount is not required. However, the pressure transfer efficiency is lower than that in the direct pressing method since the high pressure from the pressing plunger is not directly delivered to the molten metal in the metal mold cavity, but delivered through the molten metal in the runner. Thus, this results in a reduction of mechanical properties from the reduction in pressing effect. Particularly, in the case of cast products which have complicated structures and many variations in thickness, the solidification time of the molten metal in the metal mold cavity differs according to the thickness of the product. That is, while the portion of the product in the metal mold cavity with a thin thickness solidifies quickly, the thick portions solidify at a later point. Accordingly, while the portion with a thin product thickness near the runner will have already solidified, if the thick portions in the metal mold cavity are far from the runner, the pressure force from the pressing plunger will not be delivered to the incompletely-solidified thick potions. The effects of the pressure applied will not appear in the product portions with a thick thickness, causing casting defects.
To solve the above disadvantages of the indirect pressing method, Japanese patent publications Sho 49-36093 and Sho 59-30503, as shown in FIG. 5C, propose a local pressing method, wherein a pressing plunger is installed at the thick portions where the pressing effect is expected to be low, and an injection plunger is used to fill the molten metal into the metal mold cavity, afterwards forwardly pressing the pressing plunger towards the metal mold cavity, increasing the pressing effect at the portions where the pressing effect is expected to be low, thereby maximally increasing the pressure transfer efficiency. Such a local pressing method does not require an accurate poured amount of the molten metal, and the pressing effect can be applied to all portions of the cast product. Thus, this method is very useful for manufacturing cast products with complex structures and many variations in thickness.
In producing cast products using a squeeze casting apparatus using the above local pressing method, there are circumstances which require pressure to be applied from all vertical and lateral directions, especially for cast products such as housings which have complicated structures and many variations in thickness.
However, for prior art squeeze casting apparatus, aside from the lateral pressing means to effect the lateral pressing to the molten metal supplied to the metal mold cavity; other means, such as a molten metal supplying means to fill the molten metal into the metal mold cavity and the runner, and an extraction means to extract the completed cast product from the metal mold cavity, must be installed at a position vertical or lateral to the metal mold. Thus, it is difficult to install the pressing plungers which can effect lateral pressing to all vertical and lateral directions to the metal mold.